2.5.7 2.5.7.1
undergirded by a study of a number of storms and considering many factors is required in shaping the PMP isopleths.
2.5.6.5 The separation method. The reports of the D.S.
Weather Bureau (25, 32) describe a separation method of preparing generalized estimates of PMP. Both observed and probable maximum storms in the mountains near the Pacific Coast of the United States are conceived as the combined result of two effects, orographic precipitation which may be estimated from the orographic model (par. 2.4.7.4) and "convergence precipitation" from storm processes not directly resulting from the mountains. The "convergence" part of the PMP is estimated by moisture maximizing storm values occurring in relatively flat regions near the mountains. These are then
transposed to the mountains applying an assumed decrease of this component with elevation. The orographic and convergence components of the PMP are estimated for a basin separately by use of different charts and nomograms and then added together for total PMP.
The orographic and convergence components have different seasonal, areal, geographic, and elevation variations. Figures 2.24 and 2.25 depict portions of index maps of the respective components of 6-hr.
PMP in the state of California, of the D.S.A., from (32). The different character of the two distributions is evident.
Dse of generalized PMP charts
Generalized charts usually provide PlW for one or more standard duration-area combinations in map form and DDA relationships to calculate depths -for other standard duration-area combinations. From these, an array of basin-wide.6-hr. increments of PMP is obtained.
hie
PMP
f orograp - Example 0
Figure 2.24
100 ESTIMATION OJ!' MAXIMU1VI FLOODS
ence PMP f converg
Example 0
Figure 2.25
-2.5.8 Generalized charts other than PMP
2.5.8.1 Generalized charts may be constructed for categories of intense rainfall other than the probable maximum. One procedure that has been used is to construct charts of enveloped transposed rainfall values, but not maximized. The techniques are similar to those described in this chapter except that the moisture maximization step is omitted. Bailey and Schneider's charts (par. 2.5.1.2) are of this type. The Corps of Engineers, D.S. Army, uses such charts for projects where due to economic and safety considerations the design flood can be ·of a lower magnitude than the probable maximum.
These values are called "standard project" values. A summary of this has been given by Gilman (12). The degree of severity or rareness that such charts represent is greatly influenced by the number of storms available for analysis, the climate, and whether the
precipitation magnitudes are liberally or closely enveloped in constructing the isopleths on the chart.
2.6 Storm Sequences and Maximum Rainfall for ~ong Durations 2.6.1 Introduction
2.6.1.1 On the larger rivers of the world a single storm is generally insufficient to create a major flood. Rather, a period of heavy rains extending over a number of days is generally required to produce such a flood. A convenient method for simulating extreme instances of long rainy periods for design problems in a large basin is to combine two or more past rainstorms into
a sequence.
2.6.2 Aspects of storm sequence
·2.6.2.1 Composite methods. The most simple two-storm
102 ESTIF~TIOK.OF MAXIMm~ FLOODS
combination is a sequence or two storms or periods of rain that have affected the basin. The elapsed time between the storms is
shortened, perhaps from years to a few days. }lore elaborate time sequence development includes transposition of one or more storms.
Still further maximization is provided by adjustment to maximum
..
moisture of one of the storms (section 2.4.2). Procedures for constructing design storm sequences are listed in table 2.6.1 The severity of the flood corresponding to a storm sequence depends not only on the method of combining but on the severity of the individual storms. This, in turn, depends on the length of
precipitation record, and on the more or less accidental occurrence of extreme storms within the record. Therefore, no specific rules can be formulated for the type of sequence to be selected for a particular class of project. The more limited the storm sample, the farther dmvn the list of table 2.6.1 is the combination method selected.
Method 1 is sequential maximization of paragraph 2.4.1.5.
Methods 2 and 3 are spatial maximization, while 4 and 5 include both.
Under method 1 it may be possible to calculate the flood hydro graph for the sequence directly by combining the hydrographs of the respective storms by appropriate routing. All the other methods include transposition of one or more storms and therefore require calculations of runoff.
2.6.2.2 Minimum time interval. Experience shows that the length of the interval of little or no rain between t~vo storms
has an important influence on the maximum computed discharge yielded by the sequence of the two storms. Generally, reducing
by one day the intervening time between storms will produce a significantly greater overlap of the respective hydrographs and a significantly greater peak flow. Like other factors associated with design rainstorms, this minimum time interval in a hypothetical
storm sequence is derived by combination of (a) envelopment of the record - in this case selecting the smallest, not the largest -and (b) deduction to what is reasonable from the point of view of synoptic meteorology.
2.6.2.3 Dual typhoons or hurricanes. In tropical and subtropical regions subject to frequent typhoons, hurricanes, or tropical depressions at certain seasons, two of these storms in sequence should be given consideration as the prototype for a major flood over a large river basin. Tropical storm tracks are usually fairly well recorded in available publications. Study of these tracks may lead to a conclusion as to minimum reasonable time interval between two storms, or in exceptional circumstances the interval between heavy rainfalls from the same storm following a looping track.
2.6.2.4 Hypothetical map sequence technique. The most rigorous check on a presumed minimum time interval between
two storms and on the overall- synoptic compatibi-lity of the two, is to construct a series of surface weather charts depicting one possible evolution of the weather leading from one storm to the other. This is done in an illustration that follows. Other
illustrations are found in a study of the U.S. Heather Bureau (29).
Once the conviction has been established for a particular climate or region that hypothetical map sequences connecting certain